1. Field of the Invention
The present invention relates to a ceramic package for electronic parts.
2. Description of the Related Art
The recent demands for a higher density transistor, faster operation and inexpensive production costs for electronic parts, such as LSI semiconductor devices, require the ceramic packages to have the following properties: a lower dielectric constant, a higher thermal conductivity, a thermal expansion coefficient (expansivity) closer to that of a mounted semiconductor element (chip), a high mechanical strength, and a lower production cost. In addition, the ceramic packages must have a reliable hermetical sealing, encapsulation, and low .alpha.-particle radio-activity.
As a material of the ceramic package, an aluminum nitride (AlN) sintered body or a silicon carbide (SiC) sintered body has begun to replace the conventional alumina sintered body, since properties of the AlN and SiC sintered bodies are superior to those of the alumina sintered body. For example, the AlN and SiC sintered bodies have a thermal conductivity of 140 W/mK or more, which is five or more times larger than that of an alumina sintered body, and have mechanical strength similar to or greater than that of an alumina sintered body. In addition, the thermal expansivity of the AlN (or SiC) sintered body is closer to that of a silicon semiconductor element than that of the alumina body, which is a favorable property for a package material.
Nevertheless, the AlN sintered body and the SiC sintered body are very expensive compared with the alumina sintered body, and thus, the production cost of the AlN (or SiC) package is also very high. Moreover, since the AlN and SiC sintered bodies have a high dielectric constant of 8.5-10 and 10-40 at 1 MHz, respectively, signal propagation delay-time is increased. Particularly, it is difficult to apply a conventional production method, in which green sheets are laminated for a ceramic package, to the SiC sintered body.
On the other hand, use has been made of a mullite sintered body, since the thermal expansivity thereof is ever closer to that of the silicon semiconductor element. Recently, the mullite material and production process have been improved, and it is now possible to obtain a mullite package having a dielectric constant of 7.0 or less at 1 MHz, thus meeting the demand for a faster operation of electronic parts. The mullite sintered body, however, has a thermal conductivity of 10 W/mK or less, which is inferior to that of the alumina sintered body, and has a flexural strength of less than 30 kg/mm.sup.2, which is also inferior to that of the alumina sintered body. Therefore, although a mullite sintered body having a low dielectric constant and low thermal expansivity is suitable for a package on which a relatively large size semiconductor element is mounted, application of the mullite sintered body is limited due to the heat dissipation and mechanical strength thereof.